The invention relates to a method of forming a patterned photopolymer coating on a printing roller in which a removable, light-sensitive, seamless intermediate layer, applied to the photopolymer layer and adhering closely thereto, is exposed and optionally developed in a first stage, essentially to form a desired patterned opaque image in the intermediate layer, and subsequently the photopolymer layer is exposed via the image formed in the intermediate layer, after which exposure the opaque intermediate layer parts and the soluble parts of the photopolymer layer are removed.
In a known method of this type the application of the patterning film with the edges abutting or overlapping around the printing roller coated with a photopolymer layer and then wrapping a thin layer of transparent film around the whole, under which film vacuum is created so that the patterning film is pressed against the photopolymer coating under uniform pressure, are known. In this state exposure is then carried out, whereafter the wrapping film and patterning film are removed.
The patterned printing roller is produced thereafter by washing out the soluble parts of the photopolymer coating present after the exposure.
In the case of a photohardening polymer these are the unexposed parts.
In the case of a photodecomposable polymer these are, however, the exposed parts.
Unwinding the patterning film along a rotating roller coated with a photopolymer layer and exposing at the position of the contact surface through a slit are also known.
In this known method, both when photohardening and when photodecomposable photopolymer coatings are used, imperfections in the patterning occur at the point of contact of the pattern, as a result of which the printing result at the position is influenced unfavourably.
In the case of a photohardening polymer and a patterning film with overlapping edges, the photopolymer layer, which is still somewhat elastic before exposure, is somewhat dented at the position of the overlap, which dent is fixed at the position of exposed parts by hardening.
In the case of a patterning film with abutting edges the light will be scattered or reflected at the position of the edge by the transparent parts of the pattern in the patterning film, as a result of which incomplete light transmission is brought about at the position. On the other hand, at the position of the edge some light will nevertheless be transmitted through the parts not intended to transmit light (i.e. black) of the pattern in the patterning film.
Both in the case of photohardening and in the case of photodecomposable photopolymer the phenomenon occurs at the position of the abutment edge that in the case of hard parts some material may nevertheless be washed away and that in the case of soft parts which can be washed out less is removed.
In the case of unwindable patterning film and slit exposure a double exposure always takes place at the junction of the pattern as a result of the slit width of the exposure slit, as a result of which the slope formation between exposed and unexposed parts of the polymer layer is influenced and deviations are produced as a result of the slope formation for single exposure in the rest of the pattern for both photohardening and photodecomposable photopolymer coating.